It is well known in the art that images projected by the optical system of image projection apparatus may be modified in order to improve the quality of the observed picture. Such modification may be carried out by attenuating the projected light in an intermediate image plane of a single optical system, or in a plurality of optical systems in multi-channel projection apparatus. Attenuators which perform these functions may modify either the shape, or the intensity, or both, of the projected images, or of portions thereof. Collectively they may be referred to as optical attenuators, or more specifically as field stop attenuators or neutral density attenuators.
Multi-channel display apparatus is widely used where a picture having a wide field of view and improved resolution is desired. In such apparatus separate image areas are projected onto a common screen by separate optical systems. The separate images are usually juxtaposed to form a continuous, wide field of view image. For example, a domed screen may be used onto which a highly realistic image with wide panorama can be projected for training aircraft pilots.
A recurring problem in present day multi-channel display apparatus is the control by the boundaries of the separate image areas projected by the respective optical systems, such that there is no overlap and no gap between adjacent image areas on the screen. The size and shape of the image area projected to the common screen by each optical system is controlled by a field stop attenuator. A portion of the total attenuator area is apertured, the remainder being opaque. The apertured and opaque areas together determine the shape of the image on the screen. As used herein, the term "apertured" or "aperture" means an area, which may or may not be an actual hole in the attenuator, but which is capable of transmitting light substantially without attenuation. The term "opaque" refers to the ability of an area to block the transmission of light. In this application, the field stop attenuator is located in an intermediate image plane of the optical system, so as to sharply delineate the image periphery and prevent the edges of the image area from appearing faded or blurred on the screen. It will be understood that field stop attenuators may also be used in single-channel display apparatus to control the shape of a projected image.
Currently used methods for providing field stop attenuators that have apertures of the desired configuration, i.e. which are configured to avoid overlap or gaps between adjacent image areas on the screen, entail a largely iterative procedure. Initially, a field stop attenuator is provided in each channel having an aperture configured slightly smaller than that required to follow some analytical prediction. The aperture is then enlarged to the desired configuration by a process of trial and error.
For exammple, a field stop attenuator made of a brass foil, with an aperture slightly smaller than the predicted aperture configuration, may be inserted at an intermediate image plane in the optical system for each channel in a multi-channel display apparatus. Gaps between adjacent image areas, which exist because the apertures are too small, are visually determined by projecting the respective images produced by the optical systems onto a common display area or screen. The field stop attenuators are subsequently removed and their apertures are enlarged by trimming where needed. Following the re-insertion of the field stops, a further visual check is made for the existence of gaps or overlap.
This trial and error process, which requires a relatively high degree of skill to carry out successfully, continues until the desired image area boundaries are obtained, e.g. until adjacent image areas share a common boundary. The procedure is lengthy and hence expensive and prone to error. Furthermore, the technique is cumbersome and to some extent error-intolerant. For example, image overlap, which results when the predicted aperture configuration turns out to be too large, or when the aperture is erroneously enlarged too much, can be corrected only by replacing the field stop attenuator in question with one having a smaller aperture and starting the trial and error process again.
Attenuators of a different kind are used in the intermediate image plane of an optical system to provide uniformity of brightness across the projected image. Non-uniformity in the brightness of an image, as perceived by an observer, may result from the specific positional relationship of the projector and the observer, the shape and coating of the screen, as well as the lens transmission characteristics of the optical system(s). To provide an image whose brightness is more perfectly uniform, it is known to use a neutral density attenuator in order to attenuate the brighter portions of the observed image. In a commonly used prior art method for obtaining an equalized brightness flat field image, the coating on the screen is selectively altered to provide the requisite attenuation on defined areas of the screen. Such a technique is subject to inefficiency, high cost and an undue expenditure of time.